This protocol will be used to study effects of hyperglycemia upon the oxygen metabolism of the inner retina in individuals with varying severity of diabetic retinopathy. During euglycemia and hyperglycemia, blood oxygen saturation levels and blood flow rates will be measured in retinal arteries and veins perfusing temporal and nasal circulations. Blood glucose levels will be regulated with a glucose clamp technique utilizing primed continuous infusion of insulin and 20% glucose through IV catheters.